Conventionally, there is known an active matrix-type liquid crystal display device including a display unit that includes a plurality of source bus lines (video signal lines) and a plurality of gate bus lines (scanning signal lines). As for such a liquid crystal display device, conventionally, in many cases, a gate driver (scanning signal line drive circuit) for driving gate bus lines is mounted, as an IC (Integrated Circuit) chip, on the periphery of substrates forming a liquid crystal panel. However, in recent years, providing a gate driver directly on a TFT substrate (array substrate) which is one of two glass substrates constituting a liquid crystal panel has been gradually increasing. Such a gate driver is called a “monolithic gate driver”, and the like.
In the display unit of the active matrix-type liquid crystal display device, there are provided a plurality of source bus lines, a plurality of gate bus lines, and a plurality of pixel formation portions disposed at respective intersections of the plurality of source bus lines and the plurality of gate bus lines. The plurality of pixel formation portions are arranged in a matrix to form a pixel array. Each of the pixel formation portions includes: a thin film transistor which is a switching element having a gate terminal connected to a gate bus line that passes through a corresponding intersection and a source terminal connected to a source bus line that passes through the intersection; a pixel capacitance for holding a pixel voltage value; and the like. The active matrix-type liquid crystal display device is also provided with the gate driver and a source driver (video signal line drive circuit) for driving the source bus lines.
Video signals representing pixel voltage values are transmitted by the source bus lines. However, a single source bus line cannot transmit video signals representing pixel voltage values for a plurality of rows at one time (simultaneously). Accordingly, writing (charging) of the video signals to the pixel capacitances in the pixel formation portions arranged in a matrix is performed sequentially row by row. Therefore, the gate driver is configured by a shift register circuit including a plurality of stages so that the plurality of gate bus lines are sequentially selected for a predetermined period. Further, by sequentially outputting active scanning signals from the respective stages of the shift register circuit, writing of video signals to the pixel capacitances is performed sequentially row by row as described above. As used herein, a circuit that constitutes each of the stages of the shift register circuit is referred to as a “unit circuit”.
Meanwhile, regarding a thin-film transistor, a threshold shift (shifting of a threshold voltage) due to an influence of external light may occur when a bias is supplied to the gate for an extended length of time. As used herein, a term “bias” means a “constant voltage”. If a bias that keeps the thin-film transistor in an on-state (hereinafter referred to as “positive bias”) is supplied to a gate of the thin-film transistor for an extended length of time, a voltage-current characteristic of the thin-film transistor shifts to a positive direction (see FIG. 38). As a result, a threshold voltage of the thin-film transistor increases. If a bias that keeps the thin-film transistor in an off-state (hereinafter referred to as “negative bias”) is supplied to the gate of the thin-film transistor for an extended length of time, a voltage-current characteristic of the thin-film transistor shifts to a negative direction (see FIG. 38). As a result, the threshold voltage of the thin-film transistor decreases.
A unit circuit that constitutes a shift register circuit in a monolithic gate driver generally includes a large number of thin-film transistors, which include a thin-film transistor in which a bias is supplied to its gate for an extended length of time. Therefore, a threshold voltage of a part of the thin-film transistors may shift due to an influence of external light. In such a case, a power source margin changes (a range of voltages for operating the thin-film transistors changes), and it is not possible to control the on-off state of at least a part of the thin-film transistors. As a result, malfunction of the shift register circuits occurs. Therefore, in order to prevent the threshold shift due to an influence of external light from occurring, a light shielding film is often provided for a thin-film transistor in a shift register circuit.
It should be noted that the following prior arts are known in relation to the present invention. Japanese Laid-Open Patent Publication No. 2004-4553 discloses the invention related to a liquid crystal display panel having a thin-film transistor provided with a light shielding layer (light shielding film). In this liquid crystal display panel, the light shielding layer is disposed between a substrate and the thin-film transistor with insulating layers therebetween, and an off-potential to be supplied to the gate of the thin-film transistor is constantly applied to the light shielding layer. This prevents occurrence of crosstalk caused by a leak current in the thin-film transistor. Further, Japanese Patent No. 5538890 and Japanese Patent No. 5209117 disclose a variety of specific examples of a configuration of the unit circuit and a driving method of the shift register circuit.